Among secondary batteries that have been put to practical use so far, a lithium ion battery has the highest energy density and generates a high voltage. Therefore, a lithium ion battery is frequently used as a battery for laptop computers, mobile phones, and the like. Usually, the lithium ion battery is constituted with a positive electrode, an electrolyte, a negative electrode, and a separator installed between the positive electrode and the negative electrode. As the positive electrode, a material obtained by fixing an electrode mixture, which is composed of a lithium ion-containing positive electrode active material, a conduction aid, an organic binder, and the like, onto the surface of a metal foil (current collector) is used. As the negative electrode, a material obtained by fixing an electrode mixture, which is composed of a negative electrode active material that enables lithium ions to be removed therefrom or inserted thereinto, a conduction aid, an organic binder, and the like, onto the surface of a metal foil is used.
As the positive electrode active material used in the lithium ion battery, lithium-transition metal composite oxides based on lithium cobalt oxide (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4), and the like have been put to practical use.
In a case where these lithium-transition metal composite oxides are used, due to the repetition of charging and discharging, battery performance such as capacity or cycle characteristics may deteriorate. For example, although a theoretical capacity of LiCoO2 is 274 mAh/g, a battery is generally used at a voltage of about 4.2 V and a capacity of about 150 mAh/g, and accordingly, the capacity thereof utilized is only about half of the theoretical capacity. In order to obtain a higher capacity, charging and discharging need to be performed at a higher voltage, and in this case, phase transition of LiCoO2 and performance deterioration resulting from elution of cobalt markedly occur. Furthermore, there is a concern that the electrolytic solution may be oxidized and decomposed due to an increase of oxidation voltage.
LiNiO2 has a layered crystal structure just like LiCoO2, is cheap, and has a reversible capacity greater than that of LiCoO2. However, LiNiO2 undergoes a significant decrease of capacity due to a change in the crystal structure resulting from charging and discharging similarly to LiCoO2. A LiMn2O4-based active material is expected to be usable in a large-sized battery for automobiles and the like, because a source of this material is abundant, and the safety thereof is relatively high. However, Mn is more easily eluted than Co and Ni, and a capacity of LiMn2O4 markedly decreases due to charging and discharging.
In addition to the change in a crystal structure or the elution of ions described above, decomposition of the electrolyte, alteration of the surface of the active material, and the like are considered causes of decrease of the battery capacity.
As a method for ameliorating the decrease of a charging/discharging capacity of positive electrode active materials, a method of coating surfaces of the active materials with different materials is suggested. For example, NPL 1 reports that a method of coating the surface of LiCoO2 with an oxide (ZrO2, Al2O3, TiO2, or the like) is effective for suppressing a change in a crystal structure at the time of charging and discharging or elution of Co4+ ions.
Furthermore, because a positive electrode active material such as LiMn2O4 or LiFePO4 has high electrical resistance, the improvement of their performance by reducing resistance is a great challenge. As a method for address such a challenge, carbon coating is reported as an effective method. For example, NPL 2 reports that, by coating the surface of an iron phosphate-based active material with carbon, the conductivity of lithium iron phosphate is improved, and hence a battery capacity is improved.
In addition, PTL 1 discloses a method of improving a battery capacity of LiVP2O7 by carbon coating.
However, in a case of the oxide coating disclosed in NPL 1, because the transfer efficiency of electrons or lithium ions in the oxide is poor, the battery performance greatly depends on a thickness or film quality of the coating layer, and hence it is not easy to optimize the battery performance.
Furthermore, in a case of the carbon coating disclosed in NPL 2 or PTL 1, in order to form a carbon film, a high-temperature treatment process is required in general, and hence grain growth may be insufficient or costs of the active material tend to be high. In addition, by the coating method (PTL 1), in which a mixture of an active material and a carbon raw material is fired at a high temperature, a dense coating layer is not be easily obtained.